The determination of the volume of blood per unit time pumped by the heart is of importance in the diagnosis and treatment of cardiovascular disorders. For critically ill heart patients the physican must quickly and accurately determine the cardiac output in deciding whether to give the patient medication, perform surgery, or give other immediate treatment. Cardiac output measurements may also be made on a round-the-clock basis in a hospital intensive care unit by nurses or interns with only a minimum of training in the use of each specific instrument. Instruments for this purpose should be capable of accurate and rapid response with a minimum of attention required by the physician in getting the machine stabilized and ready to use. The method must be safe to the patient.
A well-known technique for the determination of cardiac output is the indicator-dilution method, wherein a measured quantity of liquid bearing some indicator is injected in the veins upstream from the heart, and the dilution of this liquid by blood is detected downstream from the heart or downstream from one part of the heart. The greater the flow of blood pumped by the heart, the faster the dilution of the indicator liquid. The injected material is most concentrated immediately after injection, and the concentration drops with the passage of time in an approximately exponential manner. This concentration at a measuring station defines a curve, and the cardiac output is determined by integrating over time the area under this curve. These curves vary in height, duration, and shape from patient to patient. Various indicators are used, including dyes which are detected optically, radioactive isotopes which are detected with radiation detectors, and in the thermodilution method liquids of different temperature from that of the blood, which are detected by thermistor temperature sensors placed in the bloodstream. Thermodilution is an advantageous indicator technique for cardiac output measurement, because the injectate may be completely non-toxic, unlike dyes and radioactive isotopes, and the thermistor measurement of temperature is relatively simple, inexpensive, and safe, and does not require removal of blood samples.